Consumer use optical disk drives are getting to be expanded in capacity by using 4.7 GB DVD media instead of the conventional 120 mm one-sided CD media having a capacity of 650 MB. And recently, the DVD media are getting to be replaced with 23 GB Blue-ray Disks (BD). The BD can record information for two hours in digital high vision quality while the DVD media can record information only for about two hours in NTSC image quality. This long-hour recording is realized by some technical improvements such as employment of shorter wavelength for semiconductor laser and high numerical aperture for objective lens. The laser wavelength is 780 nm for the CD and 650 nm for the DVD. And, for the BD, the laser wavelength reduction jumps to 405 nm (when a blue violet ray is used). Similarly, the NA is improved to as high as 0.45 for the CD and 0.6 for the DVD, and up to 0.85 for the BD in resolution. If the wavelength is assumed to be λ, the condensing spot size is proportional to λ/NA. And, when compared with the inverse number ratio of the spot area squared by the spot size, the capacity of the DVD will become 2.6 times that of the CD and the capacity of the BD will become 4.2 times that of the DVD. Actually, the capacity of the DVD is 7 times that of the CD and the capacity of the BD is 4.9 times that of the DVD. The signal processing technique such as error correction makes up for the shortage.
However, employment of such a high NA value for an objective lens to increase the capacity of the object media significantly comes to arise a side effect, that is, aberration occurrence. The aberration then causes degradation of the condensing spot due to a so-called spot fade and an increase of the spot size, which then causes the quality of recording/reproducing signals to be degraded. The objective lens is designed to condense lights optimally under fixed conditions. And, when any actual condensing state cannot satisfy the conditions, such aberration comes to occur. For example, disk inclination, improper thickness of both disk substrate and cover layer are such conditions. Of course, when the disk inclination is 0, the optimum condensation is assured. If the disk inclination is not 0, so-called coma occurs almost in proportion to the inclination angle and the substrate thickness. And, if the thickness of the disk substrate and the thickness of the cover layer are out of predetermined optimal values, aberration referred to as spherical aberration occurs in proportion to the deviation from the predetermined values. When such aberration occurs, the larger the NA value is, the more the sensitivity increases. The coma increases in proportion to the cube of the NA while the spherical aberration increases in proportion to the biquadrate of the NA. In other words, generally, the larger the NA is, the narrower the tolerance of disk inclination/thickness deviated from their predetermined values becomes.
In order to suppress the generation of such aberration partially, the disk substrate is made thin when high NA values are to be employed for the DVD and BD media. While the disk substrate is 1.2 mm for the CD media, the DVD disk substrate is reduced to 0.6 mm and the BD disk substrate is further reduced to 0.1 mm. (Actually, however, if the disk substrate is 0.1 mm, it does not function as a disk supporting member any longer. This is why, for DVD and CD media, the laser beam is not passed through the substrate to be condensed on the recording film from the back side of the substrate; the laser beam is condensed on the recording film through a 0.1 mm thick cover layer of the 1.1 mm thick substrate, which is coated at the recording film side of the substrate.) Consequently, the tolerance of BD/DVD inclination that causes coma as described above has been kept almost at the same level as that of the CD. And, because such spherical aberration often occurs in proportion to a substrate thickness deviation from its best thickness regardless of its initial substrate thickness, an increase of the NA value leads directly to the strangulation of the range of the tolerance of the substrate thickness deviation. This is why the tolerance range of the substrate thickness deviation of the BD is 3 μm, which is about 1/10 of that of the DVD, which has been 30 μm. If the BD has only one recording film layer, therefore, the BD is required to be manufactured so as to limit the cover layer thickness deviation within 3 μm. It is expected that the technical progress in recent years can satisfy the requirement, however. If a disk comes to have two recording layers formed with an interval of 20 μm and over therebetween to realize a large capacity, it is required to correct the spherical aberration to occur because of the thickness of the layer between those two recording layers (first and second) when the first layer is changed over to the second layer. In that connection, it is also required to correct the inter-layer deviation from its predetermined value, which is caused by uneven coating in a resin spinning coating process of the inter-layer film and an uneven thickness of the adhesive layer to occur in a film laminating process of the inter-layer.
A conventional technique for correcting detected spherical aberration is disclosed, for example, in WO2002/021520 (PCT application/No.JP01/007422, patent document 1). This patent document 1 discloses two methods for detecting spherical aberration. One of the methods detects a focal point at two places separately; a focal point of a beam detected around its beam axis and a focal point of a beam detected in the periphery of the beam axis. And, spherical aberration is detected from a deviation from each of the focal points. The essence of the spherical aberration is that the focal points of a beam close to the beam axis and a beam away from the beam axis are deviated from the beam axis in the front and rear direction. The conventional method detects such a deviation directly. This method can detect both of defocus error signals and spherical aberration signals in real time, so that the method can realize dynamic servo compensation of such spherical aberration.
The second method makes good use of a phenomenon that a focal point that gives the maximum amplitude to a tracking error signal (hereinafter, to be referred to as a push-pull signal) changes due to a value of such spherical aberration. A detecting method referred to as a push-pull method is used for detecting tracking error signals. The detecting method is also described in the above patent document 1. In the case of tracking error signal detection by the push-pull method, the following effect is used; the effect means that the reflection light from the disk is separated into a 0th order beam and a ±1st order diffracted beam due to a guiding groove formed cyclically on the disk and those separated beams interfere each other, thereby the interference intensity changes according to a tracking error. In other words, a two-divided photodetector is used to receive the intensity of the interference between the ±1st order beam and the 0th order beam and the intensity of the interference between the −1st order beam and the 0th order beam separately and a tracking error signal is obtained from a differential output of the photodetector. If the beam spot is positioned just on the track, those two beams are balanced, thereby the error signal takes a value of 0.
The JP-A No.351255/2001 discloses the third method. According to the method, in order to obtain push-pull signals of which focal points are defocused from the best focal point in the front and rear direction at the same time, three beams (one main beam and two sub-beams) are injected into the disk and a focus error is set in each of the two sub-beams beforehand so as to defocus it from the best focal point in the front and rear direction. And, when those sub-beams are deviated from the main beam just by ¼ of the guiding groove cycle on the disk or when a diffraction grating is used to dispose the main beam just on the track so as to obtain an effect equivalent to the above-described one practically, the push-pull signals from those two sub-beams come to have the maximum and minimum values. Consequently, when in tracking, the spherical aberration signal is obtained from a difference between the push-pull signals of the two sub-beams.
The differential push-pull method is described in, for example, in JP-A No.296875/11. Tracking error detection by the push-pull method is characterized in that consecutive tracking error signals can be detected easily from one beam spot. If the objective lens moves in the radial direction of the disk with respect to the injection beam when in tracking control, however, the beam spot formed in the photodetector also moves in combination with the objective lens sometimes to cause an offset error. In this conventional example, in order to avoid such a problem, three beam spots are condensed on the disk and two sub-spots adjacent to the main spot are deviated from the main spot just by ½ of the guiding groove cycle in the radial direction of the disk. As a result, tracking error signals of which polarities are inverted by 180° from that of the main spot are obtained from the sub-spots. And, because an offset error occurs in the main spot and the sub-spots at the same polarity respectively, a difference between the tracking error signal of the main spot and that of each of the sub-spots is calculated to enable detection of offset-cancelled tracking error signals.
[Patent document 1] W02002/021520
[Patent document 2] JP-A No.351255/2001
[Patent document 3] JP-A No.296875/1999